Conversion of aliphatic compounds to aromatics is an ongoing area of study for chemical manufacture. Certain aromatic compounds, such as para-xylene, have a relatively high commercial value. Processes that can allow formation of aromatics from feed streams that primarily have fuel value, such as natural gas or fuel gas, can be beneficial if the conversion to aromatics can be performed at a reasonable cost. Due to the relatively low activity of alkanes in processes for direct conversion to aromatics, additional improvements in processes for conversion of alkanes to aromatics are desirable.
U.S. Pat. No. 5,043,502 describes a method for dehydroaromatization of C2-C5 aliphatic hydrocarbons to form aromatics. Para-xylene is produced both from the dehydroaromatization reaction and from subsequent methylation of toluene generated by the dehydroaromatization reaction. Benzene is also produced from dehydroaromatization and is described as being methylated to toluene during the subsequent methylation.
Other conventional processes can utilize feeds containing methane. Although methane is abundant, its relative inertness has limited its utility in these conversion processes. For example, oxidative coupling methods generally involve highly exothermic methane combustion reactions, frequently require expensive oxygen generation facilities, and produce large quantities of low value carbon oxides. Non-oxidative methane aromatization is equilibrium-limited, and temperatures ≥about 800° C. are needed for methane conversions greater than a few percent.
To obviate these problems, catalytic processes have been proposed for co-converting methane and one or more co-reactants to higher hydrocarbon, such as aromatics. For example, U.S. Pat. No. 5,936,135 discloses reacting methane at a temperature in the range of 300° C. to 600° C. with (i) a C2-10 olefin and/or (ii) a C2-10 paraffin in the presence of a bi-functional pentasil zeolite catalyst, having strong dehydrogenation and acid sites, to produce aromatics. The preferred mole ratio of olefin and/or higher paraffin to methane and/or ethane in the feed ranges from about 0.2 to about 2.0.
U.S. Pat. Nos. 4,049,573 and 4,088,706 disclose conversion of methanol to a hydrocarbon mixture rich in C2-C3 olefins and mononuclear aromatics, particularly para-xylene, by contacting the methanol at a temperature of 250-700° C. and a pressure of 0.2 to 30 atmospheres with a crystalline aluminosilicate zeolite catalyst which has a Constraint Index of 1-12 and which has been modified by the addition of an oxide of boron or magnesium either alone or in combination or in further combination with oxide of phosphorus. The above-identified disclosures are incorporated herein by reference.
Methanol can be converted to gasoline employing the MTG (methanol-to-gasoline) process. The MTG process is disclosed in the patent art, including, for example, U.S. Pat. Nos. 3,894,103; 3,894,104; 3,894,107; 4,035,430; and 4,058,576. U.S. Pat. No. 3,894,102 discloses the conversion of synthesis gas to gasoline. MTG processes provide a simple means of converting syngas to high-quality gasoline. The ZSM-5 catalyst used is highly selective to gasoline under methanol conversion conditions, and is not known to produce distillate range fuels, because the C10+ olefin precursors of the desired distillate are rapidly converted via hydrogen transfer to heavy polymethylaromatics and C4 to C8 isoparaffins under methanol conversion conditions.
U.S. Publication 2013/0158324 describes a method for conversion of alkanes to heavier hydrocarbons via an alkyl halide intermediate. The types of hydrocarbons produced by the conversion reaction, including aromatics, can vary depending on the reaction conditions.
It is desirable to improve the conversion of alkanes to aromatics and ultimately increase the yield of para-xylene.